Conventionally, there is available an ultrasonic sensor that includes, as shown in FIG. 4, a hollow housing 101 having an opening on one surface thereof and a transceiver device (transceiver block) 102 having a transceiving surface for transmitting and receiving ultrasonic waves, the transceiving surface exposed on the other surface of the housing 101. The ultrasonic sensor includes a circuit board 103 arranged within the housing 101 and mounted with an electronic circuit for processing ultrasonic waves transmitted and received through the transceiver device 102 and a wiring line 104 electrically interconnecting the transceiver device 102 and the circuit board 103. The ultrasonic sensor further includes a cover member 105 provided to cover the opening of the housing 101 and a terminal 106 connected at one end to the electronic circuit of the circuit board 103 and at the other end to a power supply terminal (not shown).
When used in a motor vehicle, the ultrasonic sensor is installed in a portion highly susceptible to drenching and severe vibrating, e.g., a bumper or a front grill. In many conventional ultrasonic sensors, therefore, a filler material 107 having hydrophobicity and elasticity, such as silicone or urethane, is filled in the housing 101 accommodating the circuit board 103. It is typical that a waterproof property and a vibration resistance are obtained by filling the filler material 107 in the housing 101 in this manner (see, e.g., Japanese Patent Application Publication No. 2005-24351).
In the conventional example stated above, however, the weight and cost of the ultrasonic sensor is proportionately increased as the filler material 107 is filled in the housing 101. In addition, there is a possibility that, due to the existence of the filler material 107, distortions are generated in the circuit board 103 arranged within the housing 101. If the distortions are generated, it is likely that stresses are applied to the solder portions between the circuit board 103 and the electronic parts mounted on the circuit board 103, consequently generating cracks in the solder portions. It is also likely that the sensing area characteristics of the sensor are changed before and after filling the filler material 107. Other causes of generating distortions in the circuit board 103 include, e.g., an external load generated by the thermal expansion and shrinkage of the filler material 107. Examples of the change in the sensing area characteristics before and after filling the filler material 107 include the narrowing of a sensing area of the sensor.